Working principle of tempering furnace

Tempering furnace is a key equipment in metal heat treatment process, and its core function is to adjust the microstructure of metal materials through controllable heating and cooling, thereby optimizing their comprehensive properties such as hardness, toughness, wear resistance, etc.

1. Working principle of tempering furnace
a. Fundamentals of Thermodynamics
Core objective: To decompose quenched martensite into tempered structures (such as tempered martensite, martensite, and martensite) and eliminate quenching stress through subcritical temperature heating (below the Ac1 transformation point of the metal).
Typical case:
After quenching, the hardness of 45 steel reaches HRC62, but it is brittle; After tempering at 200 ℃, the hardness decreased to HRC55 and the impact toughness increased by three times.
W18Cr4V high-speed steel needs to be tempered three times at 560 ℃ to reduce the residual austenite from 30% to below 5%, with a red hardness (600 ℃ hardness) of ≥ HRC60.
b. Process flow
Heating stage
Rate control: ≤ 10 ℃/min (large-sized workpieces need to be heated in sections, such as holding at 100 ℃ for 30 minutes).
Purpose: To avoid cracking caused by thermal stress.
Insulation stage
Time calculation: Based on the effective thickness of the workpiece (D, unit: mm), the formula is T=K × D (K is the empirical coefficient, taken as 1.5-2.0 min/mm for carbon steel and 2.0-2.5 min/mm for alloy steel).
Temperature fluctuation: ≤± 5 ℃ (high-precision tempering furnace can be controlled within ± 2 ℃).
cooling stage
Method selection:
Air cooling: suitable for medium carbon steel and alloy steel (such as 40Cr, GCr15).
Oil cooling: Suitable for high alloy steel (such as Cr12MoV), but the oil temperature should be controlled to ≤ 80 ℃ to prevent fire.
Furnace cooling: suitable for large section workpieces (such as shafts with a diameter greater than 200mm).

2. The key technology of tempering furnace
a.atmosphere control
Vacuum tempering:
Suitable for high-precision cutting tools (such as aircraft engine blades), it can eliminate oxide scale and increase fatigue strength by 15% -20%.
Vacuum requirement: ≤ 10 ⁻ ³ Pa (1Pa ≈ 7.5 × 10 ⁻ ³ Torr).
b.Protective atmosphere tempering:
Nitrogen atmosphere: Low cost, suitable for stainless steel and mold steel, requiring controlled oxygen content ≤ 5ppm.
Ammonia decomposition atmosphere (75% H ₂+25% N ₂): Strong reducibility, can prevent decarburization, but requires explosion-proof design.

c. Temperature control
PID regulation: Temperature closed-loop control is achieved through proportional (P), integral (I), and derivative (D) algorithms, with overshoot ≤ 1%.
Multi point temperature measurement: Install 3-5 thermocouples in the furnace to ensure a temperature difference of ≤± 5 ℃ (such as a temperature difference of no more than 5 ℃ between any two points in a 2m furnace).

3. Typical applications of tempering furnaces
a. Tempering of tool steel
High speed steel cutting tools:
Process: 1270 ℃ quenching+560 ℃ x 1h x 3 times tempering.
Effect: Hardness HRC63-65, red hardness (600 ℃) ≥ HRC60, lifespan increased by 2-3 times.
Cold work mold steel:
Process: Cr12MoV steel quenched at 1020 ℃ and tempered at 180-220 ℃.
Effect: Hardness HRC58-62, impact toughness increased from 3J/cm ² in the quenched state to 15J/cm ².
b. Tempering of structural steel
Spring steel:
Process: 60Si2Mn steel is quenched at 870 ℃ and tempered at 450 ℃.
Effect: Elastic limit ≥ 1600MPa, yield to strength ratio ≥ 0.9.
Tempered and tempered steel:
Process: 40Cr steel quenched at 850 ℃+tempered at 520 ℃.
Effect: Excellent comprehensive mechanical performance (σ b ≥ 850MPa, δ ≥ 12%, psi ≥ 45%).
c. Tempering of special materials
Titanium alloy:
Process: TC4 titanium alloy solution at 950 ℃+vacuum tempering at 500 ℃.
Effect: Tensile strength ≥ 900MPa, elongation ≥ 10%, corrosion resistance increased by 30%.
aluminium alloy:
Process: 7075-T6 aluminum alloy solution at 470 ℃+aging at 120 ℃ (can be considered as low-temperature tempering).
Effect: Yield strength ≥ 503MPa, hardness ≥ 150HBW.

4. Operation precautions
Furnace loading requirements
The distance between workpieces should be ≥ 50mm, and large-sized workpieces should be hung vertically or laid flat.
Workpieces of different materials need to be processed in separate furnaces to avoid cross contamination.
Safety regulations
An oxygen concentration alarm should be installed during nitrogen tempering (alarm when O ₂ ≤ 19%).
When a circuit catches fire, it is forbidden to use water to extinguish the fire. Carbon dioxide fire extinguishers should be used instead.
Equipment maintenance
Verify the furnace temperature monthly using standard thermocouples, and adjust if the deviation exceeds ± 3 ℃.
The vacuum degree of the vacuum furnace should be checked every six months to ensure that it is ≤ 10 ⁻ ³ Pa.

5. Summary
The tempering furnace achieves directional control of metal material properties through three core technologies: precise temperature control, atmosphere protection, and process matching. In mechanical manufacturing, its application covers key components such as cutting tools, molds, springs, and shafts, directly affecting the reliability, lifespan, and cost of the product. In the future, with the development of intelligent (such as AI temperature control) and green (such as waste heat recovery) technologies, the energy efficiency and accuracy of tempering furnaces will be further improved.